Various padding devices have been employed in the past. Examples include liquid- or gas-filled bladders, e.g., water-filled cushions and pneumatic pads; and gases or liquids dispersed in a solid material, e.g., foams and gels. Generally, such padding devices operate on the principle of conformation to the shape of an object when placed under pressure. When a force, such as a person's mass, is placed on such a padding device, the device deforms so as to conform to the shape of the pressure-applying object in order to distribute the force over as large an area as possible. These devices perform adequately when the object being padded has a relatively large, uniformly shaped surface area. However, when the object being padded includes a relatively small area of concentrated force, such as that caused by a bony protuberance, the majority of known padding devices do not perform to adequately reduce the discomfort of users in many applications. This is because such padding devices exert greater responsive pressure on the area of concentrated force.
The reason for the greater pressure is that materials employed in prior art padding devices typically have a high degree of "memory." As used herein, the term "memory" will refer to that characteristic of a material in which the material returns to its original shape as a result of internal restoring forces when an external force is removed. Such materials deform to the shape of an object which applies an external force by compressing. However, due to the internal restoring forces, a pressure which is proportional to the degree of compression is exerted against the object which applies the external force. A sharp protuberance compresses the padding device more than the surrounding areas and, as a result, the padding device presses back with greater pressure in these areas of high compression. Such areas of high pressure are especially undesirable when the protuberance is a bone, such as an ankle or ischial tuberosity. The high pressure can lead to discomfort and, after periods of extended use, to actual damage to the tissue overlying the protruding bone.
The problem can be described with reference to a padding device comprising a gas dispersed in a solid material, e.g., foam. Tiny gas bubbles in foam act like millions of coil "springs." When an irregularly shaped object, such as a human body portion, exerts a force on the foam padding device, the "springs" are compressed to varying degrees, each pushing back on the body portion with a force proportional to the amount of compression. This produces differential pressures across the body portion coinciding with the padding device which in and of itself causes a certain degree of discomfort. In order to achieve intimate conformity with the human body portion, a relatively soft foam may be utilized, which can be compared to weak "springs." However, when bony protuberances exert a concentrated force on these soft foams the "springs" are greatly compressed and thus, exert larger forces against the coinciding body portion, thereby possibly causing pain and reduced circulation. Moreover, if the foam is too soft there may be total compression and thus a bottoming out effect such that the foam actually provides little or no padding in these areas. The problem exists and is even more pronounced if a stiffer foam is employed, because the "springs" are stronger and thus the forces applied back against the human body portion will be greater, particularly in areas coinciding with bony protuberance. Intimate conformity is also more difficult to achieve with stiffer foams.
Deformable silicone gel padding devices are disclosed in U.S. Pat. No. 3,449,844 by Spence, issued Jun. 17, 1969; U.S. Pat. No. 4,380,569 by Shaw, issued Apr. 19, 1983; U.S. Pat. No. 3,663,973 by Spence, issued May 23, 1972; U.S. Pat. No. 3,548,420 by Spence, issued Dec. 22, 1970; U.S. Pat. No. 3,308,491 by Spence, issued Mar. 14, 1967; U.S. Pat. No. 4,019,209 by Spence issued Apr. 26, 1977; and U.S. Pat. No. 4,668,564 by Orchard, issued May 26, 1987. In U.S. Pat. No. 4,380,569, a silicone gel containing glass microbeads is disclosed.
The silicone gel disclosed in these patents, being a cross-linked and extended chain polymer, is described as having near total memory. In other words, it returns to its original shape when an external force is removed. The internal restoring forces necessary to provide such memory are undesirable in some applications. In use, differential pressures will result depending upon the degree of deformation of the silicone gel material, with higher deformation resulting in localized areas of high pressure being exerted on the external pressure-applying object.
In order to alleviate the problem of differential pressure inherent with many prior art materials, flowable, pressure-compensating materials were developed. Such materials and applications thereof are described in U.S. Pat. No. 3,402,411 by Alden Hanson, issued Sep. 24, 1968; U.S. Pat. No. 3,635,849 by Alden Hanson, issued Jan. 18, 1972; U.S. Pat. No. 4,038,762 by Swan, Jr., issued Aug. 2, 1977; U.S. Pat. No. 4,083,127 by Chris Hanson, issued Apr. 11, 1978; U.S. Pat. No. 4,108,928 by Swan, Jr., issued Aug. 22, 1978; U.S. Pat. No. 4,144,658 by Swan, Jr., issued Mar. 20, 1979; U.S. Pat. No. 4,229,546 by Swan, Jr., issued Oct. 21, 1980; and U.S. Pat. No. 4,243,754 by Swan, Jr., issued Jan. 6, 1981. Each of these U.S. patents is incorporated herein by reference in its entirety. These patents will collectively be referred to as the "flowable, pressure-compensating material patents."
The preferred materials disclosed in U.S. Pat. No. 3,402,411 comprise from 20 to 25 weight percent polyisobutylene, from 25 to 37.5 weight percent of an inert oil, e.g. mineral oil or a saturated ester oil or a mixture thereof, and from 42.5 to 50 weight percent inorganic filler. U.S. Pat. No. 3,635,849 discloses a composition consisting essentially of from about 5 to about 45 weight percent of a polyolefin, particularly polyisobutylene, from about 15 to about 70 weight percent of a paraffin, and from about 5 to about 80 weight percent oil. Lightweight aggregate materials, for example, polystyrene beads or a heavy aggregate such as Fe.sub.3 O.sub.4 can also be added.
The flowable, pressure-compensating materials disclosed in U.S. Pat. Nos. 4,038,762, 4,108,928 and 4,243,754 include from 21.39 to 77.96 weight percent oil, 21.04 to 69.62 weight percent wax and 1 to 9 weight percent microbeads. U.S. Pat. Nos. 4,144,658 and 4,229,546 disclose flowable, pressure-compensating materials comprising 10 to 60 weight percent hollow, glass microbeads, 8.5 to 34 weight percent wax and 26.5 to 81 weight percent oil. U.S. Pat. No. 4,083,127 discloses a flowable, pressure-compensating fitting material consisting essentially of discrete, lightweight, sturdy microbeads distributed throughout a continuous phase of wax and oil.
In use, the flowable, pressure-compensating materials disclosed in the above-mentioned patents are typically placed in a pliable package or envelope to define a padding device, such as by injecting the flowable material between two leak-proof resinous sheets which are sealed at the edges. The flowable materials act hydraulically. For instance, in applications where the force being transferred to the padding device is Substantially constant (e.g., a seat cushion), flowable material in the region of the padding device coinciding with the applied force attempts to flow to other regions within the padding device away from the applied force (e.g., the flowable material is redistributed throughout the padding device to effectively equalize the pressure therewithin). Preferably, there is not a total evacuation of flowable material from the region coinciding with the applied force so that the user does not "bottom out" on the padding device and thereby experience a high force concentration and related discomfort. As a result of this migration of flowable material throughout the padding device, the applied force is distributed over a larger area, thereby reducing the pressure experienced by the user and relatedly enhancing user comfort (e.g., differential pressures throughout the padding device may be minimized by the transfer of flowable material throughout at least portions of the padding device). As can be appreciated, the larger the area over which the force can be distributed by having the padding device substantially conform to the user, through the described migration of flowable material and/or based upon a preconfigured/precontoured padding device, pressures experienced by the user on the padding device can be minimized.
Depending upon the particular padding application, the viscosity of the flowable materials can be varied to provide certain desired performance characteristics. For instance, in applications where the force applied to the padding device is more repetitive or cyclic in nature, such as in the self-reinitializing padding device disclosed in U.S. Pat. No. 5,131,174 by Drew et al., issued Jul. 21, 1992, lower viscosity flowable materials may be preferable. However, in the those padding applications in which the force is somewhat constant as described above and/or in applications where stability is an issue (e.g., where it is desirable to have the flowable material migrate/flow only when exposed to continually applied, versus instantaneously applied forces) higher viscosity materials may be used. However, increasing the viscosity of the flowable material does not decrease the ability of the flowable materials to conform to the shape of the force-applying object, only the rate at which they will migrate within the padding device, such as to achieve substantial conformance with the user to maximize force distribution. Consequently, by using high viscosity flowable materials the "reaction" or "response time" of the flowable material may be reduced, which again may be desirable for certain applications. Flowable materials are presently marketed under the trademark FLOLITE.TM. by Alden Laboratories, Inc. of Boulder, Colo. U.S.A.
In many if not all padding applications which utilize flowable materials of the above-described type, it is generally desirable to retain a certain distribution of the various constituents throughout the flowable material (e.g., a homogeneous mixture). One particular constituent which may have a tendency to separate from remaining portions of the flowable material composition are beads, such as when the beads are substantially hollow to, inter alia, reduce the weight of the flowable material composition and thus the weight of the padding device. In order to redistribute the beads in the flowable material composition, the padding device may be kneaded. Notwithstanding the commercial success of existing flowable material compositions in padding applications, such as in wheelchair seat cushions, it can be appreciated that further reduction of kneading requirements/periodicity will further enhance the potential for commercial success of these types of padding devices.
Flammability of the flowable material composition may also affect the extent of its commercial success in extending the use of flowable materials to additional padding applications. For instance, in the event that flowable material compositions are used in airplane and/or motor vehicle seat cushions, the potential exists that the padding device will be exposed to a relatively high heat source in the case of an accident. Moreover, there may be applications where there is an elevated temperature during normal use of the padding device. In order to make flowable materials commercially viable for these types of applications, it would be desirable for the flowable material and/or the envelope containing such flowable material to have a certain degree of flame retardancy.